Refinement of cellulose fiber



Patented Aug. 29, 1 933 REFENEMENT 0F CELLULOSE FIBER George A. Richter, Berlin, N. H., assignor to Brown Company, Berlin, N. H., a corporation of Maine No Drawing. Application September 6, 1930 Serial No. 480,201

6 Claima- (Cl. 92-13) This invention relates to the refinement of cellulose fiber, such as chemical wood pulp, to high alpha cellulose content in order to render such fiber a better raw material formanufacture into high grade papers and for conversion into cellulose derivatives.

In the production of refined pulps, i. e., pulps of high alpha cellulose content, it has been the practice to digest previously liberated pulps, such as sulphite or kraft, in alkaline liquors containing suflicient alkali to insure the removal of a substantial amount of non-alpha cellulose components, including lignin, beta and gamma celluloses, etc. The alkali was depended upon to insure reactlom upon and solution of the nonalpha cellulose components so that the refining operation involved a consumption of considerable alkali.

I have discovered that if unrefined, previously.

liberated pulps, such as sulphite or .kraft, are digested in water at greatly elevatedgtemperatures, for instance, at temperatures above about 175 C., considerable of their non-alpha cellu-' injure the pulp forits-intended usage. 1 have. 'further discovered, however, that the injuriousefiect of the generated organic acids may be avoided and a practically selective hydrolysis of the non-alpha cellulose components effected if a small amount of alkaliis present in the highly heated water. Evidently it is the highly heated water which is the primary refining agent, but the alkali present therein evidently inhibits the degradation of the alpha cellulose, by neutralizing the organic acids generated by the hydrolysis of non-alpha cellulose components. By applying my discoveries therefore, it is possible to effect a marked refinement of cellulose pulps with an exceedingly small consumption of alkali in comparison with the alkali usage in the processes of the prior art.

Another important advantage resulting from the digestion of cellulose fiber in highly heated water containing a small amount of alkali is that the fiber is markedly reduced in its solution viscosity, by which I mean that cellulose derivatives prepared from the fiber may be dissolved {'in suitable solvent media to produce solutions or low viscosity. Such a result is not obtainable when cellulose'fiber is'digested even at elevated temperature, say 100 C., in solutions of considerable alkalinity, as the alkali evidently retards the lowering ofthe solution viscosity of cellulose fiber. This may perhaps be best appreciated from the fact that a very drastic lowering of the solution viscosity of cellulose fiber ensues when water alone is used at temperatures of say 200 C., and that the presence of aslight amount of alkali in the water at such temperature prevents to some extent the solution-viscosity lowering effect while at the same time promoting a refinement of the fiber. The hightemperature digestion of the present invention may hence be practiced on cellulose fiber of high alpha cellulose content, 'for instance, cotton or refined wood pulp, when it is desired to produce a finished product which is to serve as a raw material in the preparation of cellulose derivatives .to be dissolved in solvent media for manufacture into such products as artificial silk, films or lacquers. The presence ofa slight amount of alkali in the highly heated water serves to preserve the high alpha cellulose content of the fiber or even to raise it to some extent. My process is, however, intended for application particularly to unrefined cellulose pulps, in which case it constitutes a refining Drocess as well as one of lowering the solution viscosity of the raw material.

When an unrefined cellulose 'pulp, such as kraft or sulphite, is used asa raw material, the pulp may be partially or completely bleached as with chlorine water or with hypochlorite liquor before the high-temperature digestion in alkaline water is effected, particularly as oxycelluloses formed during the bleaching operation are also susceptible to hydrolysis along with the other non-alpha cellulose components during the subsequent hightemperature digesting operation. Such partial or complete bleaching is of advantage, especially in the case of pulps, such as kraft, which after therefining operation may still contain suflicient lignin and other coloring matter to resist whitening in a mild or dilute bleach liquor. By carrying out a partial or complete bleaching of such pulp before the refining operation, however, it is possible to produce aproduct which after be-. ing refined may be brought toa pure white condition in a mild or dilute bleach liquor which is not conducive to the formation of much oxycellu- J lulose pulps, but also the various salts, such as the sulphites, phosphates, and carbonates, and oxides, such as zinc oxide and aluminum" oxide, enumerated in my application Serial No. 480,198 filed September 6, 1930, as these salts and oxides serve to furnish a low hydroxyl ion concentration in the water during the entire refining operation. I do not, however, claim herein specifically the use of the various chemicals enumerated in that application, which, as therein described, serve as buffers against the generation of acids in the water in which they are dissolved or suspended. The present application constitutes the parent case, which is designed to cover the digestion of cellulose fiber at temperatures above about 175 C., in water of low alkalinity to refine such pulp and/or to lower its solution viscosity, irrespective of whether such alkalinity is afforded by the presence of a small amount of. relatively strong alkali, such as caustic soda, sodium sulphide, sodium carbonate or lime, or by salts. or oxides which progressively furnish hydroxyl ions to neutralize acids formed as products of hydrolysis of the non-alpha cellulose components of the fiber. In some cases, it may be preferable to add the small amount of alkali necessary for the initial cook in slight doses to the initial cooking liquor so that the liquor is of very low alkalinity throughout the initial cooking operation.

I shall now cite an example in which an unbleached, commercial kraft pulp having an alpha cellulose content of about 89% and a solution viscosity of about, 15 was employed as a raw material as such a pulp represents a material which is more difi'icult to refine and to reduce in solution viscosity than an unbleached sulphite pulp, for example. The kraft pulp may be digested for four hours as a suitable stock suspension in an 0.1% to 0.2% solution of caustic soda at 250 to 300 C., at the end ofwhich time a product having an alpha cellulose content of about 94% and a solution viscosity of about 1.5 is produced. The digested product may then be bleached to pure whiteness, as with a hypochlorite bleach liquor, which is preferably maintained distinctly alkaline, as with caustic soda, throughout the bleaching operation in order to inhibit the formation of oxycelluloses. In order to facilitate such final bleaching operation, as well as to minimize the f"rmation of oxycelluloses thereby, the raw or unbleached pulp used as a raw material may preliminarily be subjected to the action of a suitable bleach liquor or oxidizing solution, for instance, chlorine water containing, say, 4% to,6% chlorine, based on the weight of dry pulp. Such a pretreatment results in reaction upon and solution of ligneous and other coloring, matter or renders such matter more susceptible. to subsequent hydrolysis in the alkaline water in which the high-temperature digestion is performed.

Sodium carbonate, which is a milder alkali than caustic soda, may be used in lieu of caustic soda, particularly when sulphite pulp is used as a raw material; For example, sulphite pulp in bleached or unbleached condition and having an alpha cellulose content of about 85% to 88% and a solution viscosity of about 20 maybe digested for four hours as a 5% stock suspension in a 0.1% to 0.2% solution of sodium carbonate at 175 C., at the end of which time a product having an alpha cellulose content of about 94% and a solution viscosity far below that of the raw material is produced. The advantages of using such a dilute solution at a temperature of 175 C., is perhaps best attested to by the fact that a sulphite pulp when digested for four hours as a 5% stock suspension in a' 1% solution of sodium carbonate at 100 C., yields a product of an alpha cellulose content of only about 92% and of a solution viscosity of above 10. In other words, the

temperature factor is evidently so important that a far more concentrated solution of sodium carbonate at a temperature of C., lower than that of the minimum temperature which I employ in accordance with the present invention is responsible for an appreciably lower refining action.

When a sulphite pulp is digested for four hours in a .6% to .8% solution of sodium carbonate at as high as 150 C., a product of an alpha cellulose content of not more than about 92% and a solution viscosity of about 6 is produced, once again showing that'a temperature increase of .only 25 C. is responsible for the increase in alpha the alkaline water does not have the opportunity of further refining such pulp to the same degree as when an unrefined pulp is used as a raw material. When, for example, a refined sulphite pulp having an' alpha cellulose content of about 94% and a solution viscosity of about 7, is digested for four hours as a 5% stock suspension in a 0.1% solution of caustic soda at 190 C., the product has an alpha cellulose content of about 96% and a solution viscosity of about 1.5. It is thus seen that the refined pulp undergoes a slight increase in alpha cellulose content while at the same time being markedly reduced in its solution viscosity. If the refined pulp were digested in water alone under the conditions just given, the product would have an alpha cellulose content of about 87% to 90% and a solution viscosity of about 0.2 to 0.6. These results indicate conclusively that the presence of alkali tends to inhibit a lowering of solution viscosity while at the same time maintaining or increasing the alpha cellulose content of the raw material. The process of the present invention thus depends upon the absence of a high concentration of hydroxyl ions in the water at any time-during the refining operation and'upon the heating of such.

water to a minimum temperature of about 175 C.

In carrying out the high-temperature digestions herein described, the digesters employed must be constructed to withstand pressures of about 200 to 300 pounds per square inch. The heating of the liquor may be carried out by providing a circulating system for the digester such as is employed in the so-called indirect cook in pulp making, in which case the liquor is drawn from the bottom of the digester through a screen, is passed through a heater, and returned to the top of the digester. Or, if desired, screened liquor may be removed from the top of the digester, passed through a heater, and introduced into the bottom of the digester, in order to induce a more positive circulaiton within the digester. If desired, however, the pulp and liquor may be passed through -a heater and thus do away with the necessity of screening. Where high pressure steam is unavailable for the heater, it may be desirable to bring the digester charge to as high a temperature as is possible with the available steam, and then to raise the charge to the highest temperature by a suitable electric heating unit, for instance, an electric heater strapped about the digester.

I claim:

l. A process which comprises digesting preliberated cellulose fiber at a temperature of not less than about 175 C., in water furnished with only a low concentration of hydroxyl ions to neutralize such acidity as is generated in the water.

2. A process which comprises di esting preliberated cellulose fiber of the chara ter of wood pulp at a temperature of not less than about 175 C., in water containing only a slight amount of alkali to neutralize such acidity as is generated in the water.

3. A process which comprises digesting a preliberated unrefined pulp at a temperature of not less than about 175 C., in water furnished with only a low concentration of hydroxyl ions to refine such pulp and to lower its solution viscosity.

4. A process which comprises digesting a preliberated and refined pulp of high alpha cellulose content at a temperature of not less than 175 C., in water furnished with only a low concentration of hydroxyl ions to lower primarily the solution viscosity of such pulp.

5. A process which comprises digesting at temperatures above about 175 C. a pre-liberated wood pulp ofthe character of sulphite and kraft in alkaline liquors containing in the vicinity of only about 0.1% to 0.2% of an alkali like sodium carbonate and sodium hydroxide until such pulp has acquired an alpha cellulose content of at least about 94%.

6. A process which comprises digesting at temperatures above about 175 C. a refined wood pulp having an alpha cellulose content of at least about 94% in alkaline liquors containing in the vicinity of only about 0.1% to 0.2% of an alkalilike sodium carbonate and sodium hydroxide until the solution viscosity of such pulp is only a small fraction of its original solution viscosity while the ,alpha cellulose content of such pulp is further enhanced.

GEORGE A. RICHTER. 

